Sex Allocation in Hermaphrodites

Abstract

Organisms in which individuals can reproduce both as males and females are called hermaphrodites. Sex allocation in hermaphrodites
involves the division of reproductive resources between the male and female function, and presents an interesting contrast
to species that alter sex allocation by adjusting offspring sex ratio. Theoretical and empirical research in the past four
decades have largely attempted to explain when hermaphroditism is favoured, and how sex allocation in hermaphrodites is controlled.
Furthermore, the application of endocrinological methods has elucidated some of the molecular processes that underlie hermaphroditic
sex allocation, especially in sequential hermaphrodites. Despite significant advances in our understanding of hermaphrodites,
some fundamental predictions such as the adaptiveness of changes in sex allocation have not been tested, and some basic questions
on hermaphroditism have not been answered.

Key concepts:

In hermaphroditic organisms, individuals are capable of reproducing as both males and females.

Simultaneous hermaphrodites have male and female reproductive organs that are functional at the same time, and are both involved
in a mating event.

Simultaneous hermaphroditism is favoured, when an increase in male and female reproduction yields diminishing fitness returns.

Factors such as limited mobility, low density or self‐fertilization can result in diminishing fitness returns for males, and
favour simultaneous hermaphroditism. However, local resource competition and physiological limitations could saturate fitness
through female function.

Simultaneous hermaphrodites can adjust sex allocation according to the social environment (e.g. mating group size), and the
amount of resources available for reproduction (e.g. body size).

Sequential hermaphrodites, or sex changers, reproduce as one sex for a part of their lifetime, and then switch to reproducing
as the opposite sex.

Sequential hermaphroditism is favoured in mating systems where individuals have consistently greater reproductive success
as one sex earlier and as the other sex later in life.

Sequential hermaphrodites can adjust the optimal timing of sex change according to relative size or social status in a mating
group.

There are also mixed hermaphroditic systems where individuals can repeatedly switch between sexes, or simultaneous hermaphrodites
coexist with pure sexes.

Sex change in fishes involves complex behavioural and morphological changes that are regulated by an endocrine gland system
conserved across all vertebrates.

Keywords: sex allocation; hermaphroditism; reproduction; life history

Figure 1.

Male and female fitness as a function of male allocation (fitness gain curves). In each graph, the blue curve (M) and the red curve (F) give male and female fitness gain curves, respectively. The black arrow denotes the predicted optimal sex allocation that maximizes the total fitness of a hermaphrodite through male and female functions (H). (a) The male fitness curve is strongly saturating compared to the female fitness gain curve. A female‐biased allocation is predicted. (b) Male fitness curve saturates less strongly than (a). As a result, sex allocation is less biased towards the female function. Reproduced with permission from Baeza .

A sketch of the hypothalamic‐pituitary‐gonadal axis. The hypothalamus produces GnRH, which induces the production of gonadotropins in the pituitary gland. Gonadotropins stimulate steroidogenesis in gonads, and consequently, the restructuring of gonads. Gonadal steroids may further interact with the hypothalamus and also trigger behavioural change (dashed lines) along with another hypothalamic product, AVT (see text for details). Reproduced with permission from Godwin et al..

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